1. Field of the Invention
This invention relates to a process for manufacturing highly pure chlorine dioxide by the reaction of chlorate ion, chloride ion and hydrogen peroxide in the presence of a catalyst.
2. Description of the Prior Art
Chlorine dioxide is a commercially important material in such fields as pulp bleaching and fat decoloring, and also has been used for the purpose of environmental protection and pollution abatement as in the removal of phenols from industrial sewage and in denitration of industrial waste gases.
Although chlorine is a historically established disinfectant for potable water, the formation of halogenated organics, particularly trihalomethanes, during its use has recently become a major problem. Chlorine dioxide which does not form any halogenated organics has been used as a disinfectant in the place of chlorine.
Chlorine dioxide is usually generated at points of use because it readily explodes simply upon heating or photoradiating the gaseous phase.
Thus, it is highly desirable to have a process by which highly pure chlorine dioxide can be generated economically. Further, it is desirable to have a safe process in which the generation of highly pure chlorine dioxide can be easily controlled without any danger of explosion.
One of the methods for generating chlorine dioxide is to reduce a chlorate with a reducing agent in a strong acid. The reactions which occur are exemplified below, wherein, for the sake of illustration, the chlorate used is sodium chlorate and the reducing agent is hydrochloric acid. EQU NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +NaCl+H.sub.2 O(1) EQU NaClO.sub.3 +6HCl.fwdarw.3Cl.sub.2 +NaCl+3H.sub.2 O (2)
Reaction (1) must be accelerated in order to reduce the production of the chlorine. A method to achieve this purpose is to use a catalyst which accelerates reaction (1) in preference to reaction (2). However, if reaction (2) does not proceed, the purity of the chlorine dioxide ##EQU1##
In order to highly purify the chlorine dioxide product, the chlorine gas accompanying the chlorine dioxide must be eliminated from the resulting material. The substantial elimination of the chlorine gas therefrom is very difficult, resulting in the process being costly.
With reference to the catalysts for chlorine dioxide production, complex catalysts consisting of palladium(II) and an amino acid or an alkali metal salt thereof, palladium(II) and a .beta.-diketone, and palladium(II) and chlorine ion were described in U.S. Pat. Nos. 4,154,810; 4,051,229; and 4,178,356, respectively.
One of the methods for generating highly pure chlorine dioxide is described in U.S. Pat. No. 4,250,159. In this method, chlorine dioxide is generated by reducing sodium chlorate with sulfur dioxide in accordance with the equation: EQU NaClO.sub.3 +1/2SO.sub.2 .fwdarw.ClO.sub.2 +1/2Na.sub.2 SO.sub.4( 3)
As reaction (3) is slow, chlorine dioxide should be generated at a high acidity of 9.5 to 11 normal and an effluent containing a high concentration of sulfuric acid should be withdrawn. Further, this method is inefficient because the conversion of sodium chlorate to chlorine dioxide is low. Also, the purity of the chlorine dioxide generated is low, because of the poor reactivity of sulfur dioxide. Furthermore, this method may require complicated instrumentation for vaporizing sulfur dioxide.